ref value in translation candidates module element <mtc:match>

Hello team,

I have an XLIFF
specification question about 'ref' attribute used by <mtc:match>
element.

http://docs.oasis-open.org/xliff/xliff-core/v2.1/csprd01/xliff-core-v2.1-csprd01.html#candidates_ref

'ref' attribute
value is IRI, pointing to a span of text within the same unit, according
to this specification. However, I don't understand what is right way to
point a span of text with this description.

http://docs.oasis-open.org/xliff/xliff-core/v2.1/csprd01/xliff-core-v2.1-csprd01.html#matchAnnotation

Above specification
is explaining use of <mrk> element to annotate a translation candidate.
(BTW, this explanation does not look great, because it does not mention
<mrk> element directly at all, and just specifying what attributes
are used there. It should state <mrk> element is used for the annotation
first.)

My question is
whether a value 'ref' attribute in <mtc:match> must be pointing a
<mrk> element under the same <unit> using the XLIFF fragment
identification mechanism always.

I understand the
use case example in 5.1.4 Translation Candidate Annotation below is a typical
use case.

-----------------
<unit id="1">
  <mtc:matches>
   
<mtc:match ref="#m1">
   
  <source>He is my friend.</source>
   
  <target>Il est mon ami.</target>
   
</mtc:match>
   
<mtc:match ref="#m1">
   
  <source>He is my best friend.</source>
   
  <target>Il est mon meilleur ami.</target>
   
</mtc:match>
  </mtc:matches>
  <segment>
   
<source><mrk id="m1" type="mtc:match">He
is my friend.</mrk></source>
  </segment>
  <segment>
   
<source>Yet, I barely see him.</source>
  </segment>
</unit>
-----------------

What I don't understand
is - the example in 4.3.1.22 isolated. I pasted the example below:

-----------------
   <file
id="f2" xmlns:abc="urn:abc">
   <unit
id="1">
   
 <mtc:matches>
   
   <mtc:match id="tc01" ref="seg2">
   
     <source><sc id="1" isolated="yes"/>Warning:</source>
   
     <target><sc id="1" isolated="yes"/>Attention
:</target>
   
   </mtc:match>
   
 </mtc:matches>
   
 <segment id="seg2">
   
   <source><pc id="1">Warning: File not
found.</pc></source>
   
 </segment>
   </unit>
 </file>
-----------------

This example is
used for explaining <sc> ans <ec>, and includes <mtc:matches>.
In this example, you can see <mtc:match id="tc01" ref="seg2">.
I think there are two errors in this example.

The first issue
is, the value of ref attribute "seg2" is not a valid XLIFF fragment
identifier. According to the spec (http://docs.oasis-open.org/xliff/xliff-core/v2.1/csprd01/xliff-core-v2.1-csprd01.html#fragid),
it should have prefix '#' always.

The second issue
is, I think the intent of this example is to point the text span '<pc
id="1">Warning: File not found.</pc>' in <source>
element in <segment id="seg2">. Pointing <segment>
element does not look right, because it's not a span of text. Or, are we
able to use <mtc:match> to point to a <segment> element?

If 'ref' attribute
in <mtc:match> can only point to <mrk> element with type="mtc:match",
then I think section 5.1.6.5 ref should clearly state the value must point
to <mrk> element (with type="mtc:match") within the same
unit.

If 'ref' attribute
can point to any other elements, although it sounds weird, it should explain
what element can be used for specifying a span of text, and should provide
an extra example.

Anyway, can someone
clarify the original intent, and precise definition of 'ref' attribute
value in <mtc:match> element?

Thanks,
Yoshito